66
3 Heterogeneous and Homogeneous Catalytic Partial Oxidations …
been realized using polymeric materials with metal ligands [88] and has been developed for heterogeneous catalysis. A shared triazine-based framework (CTF) via the
trimerization of 2,6-dicyanopyridine in molten ZnCl 2 has also been developed [89].
This solid ligand modulates PtCl 2 to form a solid similar to the bipyrimidine PtCl 2
system. The Pt–CTF system was activated by the oxidation of methane in fuming
sulfuric acid to give methyl bisulfate. The catalyst required an activation period, but
eventually reached a TON of greater than 250. These examples demonstrate how
concepts developed in homogeneous catalysis can be useful for the development of
heterogeneous catalysts. However, in the context of industrial application, this batch
process showed very low space–time yield and required the post-treatment of the
intermediate methyl hydrogen sulfate and re-oxidation of SO 2, which can be prohibitively expensive. Additionally, the loss of the Pt-CTF catalyst in each recycling
step (about 5–10 wt%) is cost-inefficient.
3.6 Summary
This chapter discussed the direct conversion of methane to methanol over homogeneous or heterogeneous catalysts. In these catalytic reactions, the yields of the
methanol obtained are low [90], mainly due to the high reactivity of methanol under
the reaction conditions for the activation of methane. In other words, the current state
of the art does not allow the simultaneous maximization of both methane conversion
and methanol selectivity. The available reactions give high methanol selectivity only
at very low conversions and are not suitable for production on industrial scales.
This chapter also reviewed the conversions of methane to methyl esters in homogeneous systems and the conditions for achieving a tradeoff between conversion
and selectivity. These reactions achieved very high product selectivity even at high
methane conversion [91], and also allow the production of methyl esters in a concentration range of 2 mol L
−1 or higher [92], which is the desired product concentration
necessary for achieving product separation and recovery on an industrial scale.
Methanol production at this concentration has not been achieved with conventional
heterogeneous catalysts, and this high yield of methanol derivative production is
due to the conversion to methyl ester, which is less reactive than methanol under
methane conversion reaction conditions.
Since the methyl ester obtained from methane must be hydrolyzed and converted
to methanol, some reports have addressed the production of methyl esters as methanol
production. However, the approaches for the production of methanol from methane
via a methanol derivative and those for converting methane directly to methanol
should be considered separately. The hydrolysis process is required for obtaining
methanol from methanol derivatives. The methyl bisulfate or methyl trifluoroacetate
hydrolysis is relatively easy on a small scale. However, on an industrial scale, several
other challenging factors, such as the exothermic addition of the reaction mixture to
water which needs to be performed slowly to prevent the volatilization of methanol,
must be considered [90]. The process for producing methanol from methane via
3 Heterogeneous and Homogeneous Catalytic Partial Oxidations …
been realized using polymeric materials with metal ligands [88] and has been developed for heterogeneous catalysis. A shared triazine-based framework (CTF) via the
trimerization of 2,6-dicyanopyridine in molten ZnCl 2 has also been developed [89].
This solid ligand modulates PtCl 2 to form a solid similar to the bipyrimidine PtCl 2
system. The Pt–CTF system was activated by the oxidation of methane in fuming
sulfuric acid to give methyl bisulfate. The catalyst required an activation period, but
eventually reached a TON of greater than 250. These examples demonstrate how
concepts developed in homogeneous catalysis can be useful for the development of
heterogeneous catalysts. However, in the context of industrial application, this batch
process showed very low space–time yield and required the post-treatment of the
intermediate methyl hydrogen sulfate and re-oxidation of SO 2, which can be prohibitively expensive. Additionally, the loss of the Pt-CTF catalyst in each recycling
step (about 5–10 wt%) is cost-inefficient.
3.6 Summary
This chapter discussed the direct conversion of methane to methanol over homogeneous or heterogeneous catalysts. In these catalytic reactions, the yields of the
methanol obtained are low [90], mainly due to the high reactivity of methanol under
the reaction conditions for the activation of methane. In other words, the current state
of the art does not allow the simultaneous maximization of both methane conversion
and methanol selectivity. The available reactions give high methanol selectivity only
at very low conversions and are not suitable for production on industrial scales.
This chapter also reviewed the conversions of methane to methyl esters in homogeneous systems and the conditions for achieving a tradeoff between conversion
and selectivity. These reactions achieved very high product selectivity even at high
methane conversion [91], and also allow the production of methyl esters in a concentration range of 2 mol L
−1 or higher [92], which is the desired product concentration
necessary for achieving product separation and recovery on an industrial scale.
Methanol production at this concentration has not been achieved with conventional
heterogeneous catalysts, and this high yield of methanol derivative production is
due to the conversion to methyl ester, which is less reactive than methanol under
methane conversion reaction conditions.
Since the methyl ester obtained from methane must be hydrolyzed and converted
to methanol, some reports have addressed the production of methyl esters as methanol
production. However, the approaches for the production of methanol from methane
via a methanol derivative and those for converting methane directly to methanol
should be considered separately. The hydrolysis process is required for obtaining
methanol from methanol derivatives. The methyl bisulfate or methyl trifluoroacetate
hydrolysis is relatively easy on a small scale. However, on an industrial scale, several
other challenging factors, such as the exothermic addition of the reaction mixture to
water which needs to be performed slowly to prevent the volatilization of methanol,
must be considered [90]. The process for producing methanol from methane via